Question

Assuming I have two methods. One that creates an arraylist and returns it, the other retrieving that list to process. both methods are in different classes:

public ArrayList getSocialUsers() throws SQLException
{
    Connection conn=DriverManager.getConnection("jdbc:mysql:///socialsystemtest1?user=root&password=");
    Statement stmt= null;
    String query="SELECT FORE_NAME FROM socialsystemtest1.socialworkers";
    stmt= conn.createStatement();
    ArrayList<String> socialWorkers= new ArrayList<String>();
    try{
        ResultSet rs= stmt.executeQuery(query);
        while(rs.next())
        {
            String user= rs.getString("Fore_Name");
            socialWorkers.add(user);
        }
    }
    catch(SQLException e) {
        //do nothing
    }
    finally{
        if(stmt != null) {
            stmt.close();
        }
    }

    return socialWorkers;

}

public void processArray(){
  try{
    ArrayList<String>= db.getSocialUsers();
    //code ommitted
  }
  catch(SQLException e){
    //deal with it here
  }
}

db is an instance of the class that returns the arraylist already instantiated. I recieve a null pointer error at the first line of the second classes method: db.getSocialUsers();

when calling the getSocialUsers() method on its own this works and an ArrayList is indeed returned with the correct elements but when calling it from another class gives a null pointer and I don’t know why

Answer 1

This means that the db object is null, not the ArrayList. Where are you instantiating it?

Edit: I recommend you replace the instantiation in the constructor with a member initialization:

public class Admin {
    Database db = new Database();
    // etc.
}